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What Is an Underdrive Pulley?



    • Auto mechanics use underdrive pulleys as a replacement pulley that contains a different ratio and material specifications that free up lost horsepower. By using an underdrive pulley, you are increasing the diameter of each accessory pulley and decreasing the ratio of the crank pulley. The speed at which the accessories are turning relative to the engine's rpm's decrease.


    • Rather than adding horsepower to the engine, the underdrive pulley frees up the power generally wasted on overdriven accessories. A larger pulley will drive the accessories slower and reduce their parasitic horsepower loss. So you do not gain anymore power total, but you do send more power to the wheels.


    • By having a reduction in wearing (using less) out the accessories (due to the lower duty cycles), you can actually extend the life of these parts. In other words, car parts such as the alternator, power steering, water pump and air compressor are using horsepower from the engine. If you replace factory sized pulleys with an underdrive pulley, the accessories are slowed, which gives you more horsepower for the wheels.


    • Underdrive pulleys have been around since the 1950s, but they didn't gain the attention of mainstream car enthusiasts until the mid-to late 1980s. In the 1950s, they were only seen in race cars. The pulleys were used in these cars to prevent the accessories from being driven too hard. The race cars were driven at such high speeds for long periods of time. The accessories were usually damaged during the time of high-speed races, so mechanics would use the underdrive pulleys to bring some relief to the car. In the 1980s, when the single serpentine belt was introduced, the idea of using underdrive pulleys to enhance the performance of street cars enabled underdrive pulleys to become popular.


    • The underdrive pulleys have not been known to cause any issues or conflicting effects at all to an automobile. Most users of underdrive pulleys find that installing the pulley is a straightforward swap of their original pulley, and it has given their car more horsepower.

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